A human-factors expert will join the investigation of the fatal crash of Virgin Galactic's passenger spacecraft to study why the co-pilot prematurely unlocked a pivoting tail section of the ship during a test flight, a top safety official said on Monday.

The untimely engagement of the tail mechanism, designed to slow the vehicle's descent into the atmosphere from space, and the possibility that pilot error was to blame, were disclosed by the National Transportation Safety Board (NTSB) late on Sunday and have emerged as a main thrust of the inquiry into Friday's crash.

“We know already from having the lever move from lock to unlock that we need to get a human-factors person in here,” said NTSB Chairman Christopher Hart. “The question then is why did that happen when it happened?”

Investigators have yet to determine whether releasing the tail mechanism too early caused or contributed to the crash of the space plane near the Mojave Air and Space Port, about 95 miles (150 km) north of Los Angeles, Hart said in an interview.

The suborbital rocket vehicle dubbed SpaceShipTwo broke into pieces over California's Mojave Desert and crashed shortly after its separation from the special jet aircraft that carries it aloft for its high-altitude launches.

The crash, which unfolded without SpaceShipTwo catching fire or exploding in flames, came on its fourth powered test flight, the first since January.

Video footage from the cockpit shows co-pilot Michael Alsbury, 39, who died in the crash, releasing a lever to unlock the twin-tail section eight seconds after SpaceShipTwo's engine ignited, Hart told reporters on Monday night.

Two seconds later the tail, which did not have sufficient aerodynamic pressure to hold it in place, began to pivot upward, a maneuver designed to increase drag for atmospheric re-entry.

Mystery of pilot's survival

Investigators also are trying to determine how surviving pilot Pete Siebold, 43, managed to get out of the rocket plane and parachute to the ground from a height of roughly 50,000 feet, an altitude virtually devoid of oxygen.

Hart said Siebold, now hospitalized with a shoulder injury, did not exit through the cockpit's escape hatch. “We know it wasn't through there, so how did this pilot get out?” he said.

SpaceShipTwo, developed by the fledgling space tourism company of billionaire entrepreneur Richard Branson, was designed to carry wealthy passengers on short rides into space, with Virgin Galactic planning to begin offering its first flights to paying customers next spring.

The crash came three days after the unmanned rocket of another private space company, Orbital Sciences Corp, exploded during liftoff from a commercial launch pad in Virginia on a mission, under contract with NASA, to deliver cargo to the International Space Station.

Hart told a news conference on Sunday that investigators had determined SpaceShipTwo's tail system was supposed to have been released for deployment as the craft was traveling about 1.4 times the speed of sound. Instead, the tail section began pivoting when the vehicle was flying at Mach 1, the speed of sound.

“I'm not stating that this is the cause of the mishap. We have months and months of investigation to determine what the cause was,” Hart said.

Asked if pilot error was a possible factor, Hart said: “We are looking at all of these issues to determine what was the root cause of this mishap ... including that possibility.”

Also unclear was exactly how the tail mechanism began to rotate once it was unlocked, since that maneuver requires a separate pilot command that was never given, Hart said.

This raised questions about whether the craft's position in the air and its speed somehow enabled the tail section to swing free on its own.

About 800 people have paid or put down deposits for a ride into space at $250,000 a seat. Branson plans to be on the first commercial flight with his son.

Branson said Monday his company's venture was “absolutely” worth the risks. “It's a grand program, which has had a horrible setback, but I don't think anybody ... would want us to abandon it at this stage,” he told NBC.